1. Field of the Invention
This invention relates to zoom lenses suited to television cameras, photographic cameras, or video cameras and, more particularly, to large relative aperture, high range zoom lenses whose F-number for the wide-angle end is 1.6 and whose range is 8 to 44, wherein good stability of high optical performance is maintained over the entire focusing range, as focusing is performed by a lens subunit constituting part of the first lens unit, or by employing the so-called inner focus type.
2. Description of the Related Art
In the recent state of art of color television cameras for broadcasting, quick and easy handling and good manageability are regarded as important. To meet such a demand, even the image pickup devices are reduced in size. So, the use of CCD (solid-state image sensor) of reduced size of, for example, 2/3, or 1/2 inch has been coming to be the mainstream. By this, it is then insured to minimize the bulk and size of the whole camera apparatus.
In response to this demand, another measure has been taken to the zoom lens for use in the color television camera for broadcasting by reducing its size and weight. At the same time, the capabilities of the zoom lens are heightened.
Of the zoom lens types, one comprising, from front to rear, a focusing or first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of positive or negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power for forming an image, or the so-called 4-unit zoom lens, has found its use in many color television cameras for broadcasting, since it is comparatively easy to increase the zoom ratio and the relative aperture.
In recent development of zoom lenses for broadcasting, there are strong demands for shortening the focal length in the wide-angle end, or elongating the focal length in the telephoto end, and for increasing the zooming range. In addition to these, how much ahead of the television camera a close object to be shot may take its place has come into consideration. In the zoom lens that is adapted to be used in the television camera for broadcasting, therefore, a shortening of the so-called MOD (Minimum Object Distance) is becoming one of the important factors on the design specification and also on picture effects.
However, if one attempts to shorten this MOD, the variation with focusing of all aberrations becomes a serious problem. Particularly in spherical aberration, astigmatism and chromatic aberrations, the variation gets larger, causing the optical performance to be lowered extremely. With regard to this result of aberration variation by focusing, the longer the focal length and the smaller the F-number, or the faster the lens system and the shorter the MOD, the more prominent the aberration variation becomes. For this reason, many technical ideas have been made in the method of focusing the zoom lens.
Of the focusing types, there is one in which the focusing provision is made, not in the front or first lens unit, but in an inner portion of the lens system, or the so-called "inner focus" type. Using this type, many proposals have previously been made.
Such an inner focus type zoom lens is exemplified in, for example, Japanese Patent Publication No. Sho 52-41068, as obtained by applying it to the socalled 4-unit zoom lens that comprises, from front to rear, a first lens unit of positive refractive power, a second lens unit of negative refractive power for varying the focal length, a third lens unit of negative refractive power for compensating for the image shift with zooming and a fourth lens unit of positive refractive power. In this case, the first lens unit is made movable in part on the image side as a lens subunit for focusing.
Meanwhile, Japanese Patent Publication No. Sho 59-4686 discloses a technique that the first lens unit is divided into three lens subunits, of which the first is negative in refractive power, the second is positive, and the third is positive, wherein as focusing goes from an infinitely distant object to an object at the minimum distance, the second lens subunit is moved toward the image side.
In general, the zoom lenses of the inner focus type have features that, as compared with that type of zoom lens which moves the first lens unit as a whole, the effective diameter of the first lens unit gets small, it becomes easy to minimize the size of the entire lens system, and close-up photography, particularly photomacrography, becomes easy to do, and further quick focus adjustment becomes possible, because the lens unit for focusing is relatively small in size and light in weight, so that a weaker force is sufficient to drive the focusing lens unit.
However, an adverse effect is caused so as to increase the variation with focusing of aberrations, particularly spherical aberration in the telephoto end, giving rise to a very difficult problem of obtaining high optical performance maintained stable over the entire focusing range with the limitation of the size of the entire lens system to a minimum.
FIG. 15 and FIG. 16 are schematic diagrams used to explain such variation with focusing of spherical aberration in the telephoto end.
In FIGS. 15 and 16, the 4-unit zoom lens has its first lens unit F made constructed with a first lens subunit F11 of negative refractive power, a second lens subunit F12 of positive refractive power and a third lens subunit F13 of positive refractive power, totaling three lens subunits, wherein the second lens subunit F12 is moved to effect focusing.
When focused on an infinitely distant object, the first lens unit F and a second lens unit V take a paraxial power arrangement as shown in FIG. 15.
In this figure, a light beam RL1 is axial at the F-number in the telephoto end, and another light beam with a principal ray RL2 comes from the maximum angle of view-field in the wide-angle end.
With the axial light beam in the telephoto end, whilst when focused on an infinitely distant object, it travels as shown by RL3 in FIG. 16, and when focused on an object at the minimum distance, it changes to a light beam RL4.
As is apparent from FIG. 16, the height of incidence on the first lens subunit F11 is lower, and the height of incidence on the second lens subunit F12 is higher when on an object at the minimum distance than when on an infinitely distant object. For this reason, the spherical aberration varies in the negative direction as focusing goes down.
Here, to assure reduction of the size of the first lens unit F, as is understandable from FIG. 15, there is need to minimize the outer diameters of the first and second lens subunits F11 and F12 in order to admit of the principal ray RL2 of the maximum angle of field coverage in the wide-angle end.
To this end, it is necessary either to reduce the inclination of the principal ray of the maximum angular field in the travel from the third lens subunit F13 to the second lens subunit F12, or to put the rear principal point of the first lens unit F to a more rearer position so as to reduce its spacing with the variator V. The height of incidence is thus lowered.
This could be achieved if the refractive power of the third lens subunit F13 were strengthened. However, at the same time, the degree of divergence of the F.sub.NO ray in the telephoto end would be caused to increase, as can be seen from FIG. 15.
Further, if one attempts to shorten the minimum object distance, an increased total movement of the second lens subunit F12 results. To allow this, the separation between the second and third lens subunits F12 and F13 for an object at infinity is necessarily increased greatly. Hence, much reduction of the size would become even more difficult to perform.
Since, as described above, the simultaneous fulfillment of the requirements of reducing the size and of shortening the minimum object distance results in an increase of the divergence of the F.sub.NO ray for the telephoto end in between the second and third lens subunits F12 and F13, and an increase of the total focusing movement of the second lens subunit F12 at once, the heights of incidence of the rays of FIG. 16 on either of the first and second lens subunits F11 and F12 differ more greatly from each other, with the result that the spherical aberration in the telephoto end varies to a far larger extent with focusing.
Referring again to the above-mentioned Japanese Patent Publication No. Sho 52-41068, the focusing method proposed herein does not always make sure that the lens configuration is adequate for zoom lenses for broadcasting under the demands of high specifications such as large relative aperture, high zoom ratio, short MOD, etc. Particularly with the first lens unit, the shares of its refractive power between the focusing and stationary parts thereof, and the values of dispersion of the glasses of the constituent lens elements are not always sufficiently suited to the high class of zoom lens for broadcasting.
In general, to achieve fulfillment of the recent user's demand, that is, heighten the specifications of the zoom lens in such a manner that the bulk and size and the weight of the entire system are minimized, it is necessary to set forth proper features for the refractive power of all of the lens units and the construction and arrangement of the constituent lenses. Particularly in designing a 4-unit zoom lens, since the first lens unit most affects the bulk and size and the weight of the entire lens system, the proportion of the refractive power it shares, and its speed are important factors. What balance of them to make relative to the whole zoom lens becomes an important element of decision.
Furthermore, to obtain high optical performance throughout the entire focusing range, variation with focusing of aberrations must be suppressed. Of these, spherical aberration and chromatic aberrations vary to the largest extent with focusing. If the variation of these aberrations fails to be as far suppressed as possible, good image quality can no longer be obtained. For this reason, how to make the focusing lens unit and the stationary lens units share aberrations and contribute to achromatism becomes another important element.